A low temperature polysilicon thin film transistor (LTPS-TFT) possesses better electrical performance than an amorphous silicon (a-Si) thin film transistor. The size of an LTPS-TFT may be smaller than that of an a-Si TFT, and thus light penetration rate can be increased, which in turn reduces the load of the backlight module of a liquid crystal display panel and extends the service life of the liquid crystal display panel. Furthermore, since a low temperature polysilicon film (LTPS) can be processed to form a high speed CMOS (Complementary Metal Oxide Semiconductor) driver circuit system on a substrate directly, which allows less pins for an external printed board and less connection points for wirings, lowering the probability of defects in the liquid crystal display panel and increasing the endurance.
In a low temperature polysilicon thin film transistor, a polysilicon thin film is used for an active layer. For the prior art technology, in the process of forming the polysilicon thin film active layer, an amorphous non-crystalline silicon thin film is first deposited as a precursor film, and then the precursor film is crystallized into a polysilicon film with e.g., excimer laser annealing. However, in this method, the pulsed laser generated by the excimer laser has a short pulse width and melting time thus obtained is of only tens of nanoseconds, and therefore the crystallization rate is fast, resulting in grains of small sizes, which tends to generate many grain boundaries in the channel, hence reducing carrier mobility and increasing leakage current. In addition, since an amorphous non-crystalline silicon thin film is used as the precursor film, and the non-crystalline silicon still has a high melting point, while the energy for laser crystallization is limited in a certain rage, and in case of low energy, non-crystalline silicon that can be completely molten concentrates in the superficial layer. For underlying layer, the temperature is lower than the melting point for crystallizing silicon, and therefore it exhibits a semi-molten state. The direction of crystallization will be growing upwards from the molten seed crystal, and the resulted polysilicon appear in columns, which further impacts improvements of the carrier mobility. However, if the energy density of incident laser is raised, it is likely to cause non-uniform crystalline particles with obvious bumps, which will impose disadvantageous influence on subsequent film deposition.